VEGF is a central regulator of angiogenesis, and cancer is dependent upon this process since tumors cannot grow past a microscope size without the generation of new blood vessels to provide a sufficient nutrient supply. The VEGF system is thus an attractive therapeutic target for disease such as breast and uterine cancer, which require angiogenesis for growth and metastasis. Little is known about the regulation of VEGF expression in hormone related cancers such as those of the mammary gland and endometrium, and it is unclear how one might prevent VEGF expression or actions in these diseases. We have been involved in studying the hormonal regulation of VEGF expression in these tissues, and recently made the novel discovery that progesterone regulates expression of this key angiogenic factor in some human breast cancer cells, specifically in the well characterized T47-D cell line. The aim of this proposal is to determine the molecular mechanisms involved in the regulation of VEGF by progesterone in human breast cancer cells, in normal rodent mammary cells in vivo, in several in vivo mammary tumor models and in human xenografts grown in nude mice. The hypothesis we will test is that progestins (P) regulate transcription of VEGF directly via classical progesterone (PR) mediated signaling pathways. To test this hypothesis we propose 4 specific aims: (1) To determine the mechanism by which progestins regulate expression of VEGF in human breast cancer cells, (2) To determine which form(s) of the PR regulates VEGF expression, the receptor domains necessary for regulation, and the regulatory elements in the VEGF gene that mediate PR induction, (3) To define VEGF regulation by progestins and anti- progestins in vivo in the normal rodent mammary gland, in rodent mammary tumor models and in human tumor xenografts grown in nude mice and (4) to determine if PO induced VEGF in breast cancer cells participates in the process of angiogenesis and tumor growth. Elucidating the mechanisms that control VEGF production in normal and neoplastic mammary cells is essential to understand the regulation of angiogenesis in breast cancer and to design new therapeutic approaches to this disease that modulate VEGF-induced angiogenesis.